no transparency in cycles render
If your PNG has a transparent background, plug the Image Texture with your image in the bottom entry of a Diffuse, plug this Diffuse in a Mix Shader, and plug the Mix Shader in the Material Output. Now plug a Transparent BSDF (100% white) in the top entry of the Mix Shader, and plug the Alpha output of the Image Texture in the Factor of the Mix Shader. It should work